Human serum albumin (hereunder also referred to as “HSA”) is a principal protein component present in the plasma, consists of a single chain polypeptide comprising 585 amino acid residues and has a molecular weight equal to about 66,000 Dalton (see Minghetti, P. P. et al. (1986), Molecular structure of the human albumin gene is revealed by nucleotide sequence within 11-22 of chromosome 4. J. Biol. Chem. 261, pp. 6747-6757). It has been known that the principal roles of HSA are not only to maintain the normal osmotic pressure of the blood, but also to bind with a variety of substances such as calcium ions, fatty acids, bilirubin, tryptophan and drugs possibly present in the blood, thereby playing a role of a carrier for transporting these substances. Purified HSA has been used in, for instance, the postoperative treatment after surgical operations and the treatment of hypoalbuminemia caused due to the loss of albumin such as hemorrhagic shock, burn and nephrotic syndromes.
Conventionally, HSA has been prepared by subjecting the human plasma to the low temperature ethanol-fractionation method of Cone or any method similar thereto to give HSA-containing fractions (HSA is fractionated in the fraction V) and then purifying the fraction through the use of a variety of purification techniques. Moreover, there has recently been developed a method in which the human plasma is not used as a raw material, for instance, a technique for producing human serum albumin using yeast, Escherichia coli or Bacillus subtilis cells, while making use of the gene recombination technique.
These gene recombination techniques are detailed in (1) Production of recombinant Human Serum Albumin from Saccharomyces cerevisiae; Quirk, R. et al. Biotechnology and Applied Biochemistry, 1989, 11: 273-287, (2) Secretory Expression of the Human Serum Albumin Gene in the Yeast, Saccharomyces cerevisiae; Ken Okabayashi et al. J. Biochemistry, 1991, 110: 103-110, (3) Yeast Systems for the Commercial Production of Heterologous Proteins; Richard G. Buckholz and Martin A. G. Gleeson, Bio/Technology, 1991, 9: 1067-1072 for the yeast, (4) Construction of DNA sequences and their use for microbial production of proteins, in particular, human serum albumin; Lawn, R. M. European Patent Publication No. 0073646A (1983), (5) Synthesis and Purification of mature human serum albumin from E. coli; Latta, L. et al. Biotechnique, 1897, 5: 1309-1314 for the Escherichia coli (E. coli), (6) Secretion of human serum albumin from Bacillus subtilis; Saunders, C. W. et al. J. Bacteriol. 1987, 169: 2917-2925 for the Bacillus subtilis. 
The methods for purifying the human serum albumin usable herein in general include those currently used in the protein chemistry such as a salting out method, an ultrafiltration method, an isoelectric precipitation method, an electrophoresis method, an ion-exchange chromatography technique, a gel filtration chromatography technique and/or an affinity chromatography technique. Indeed, the human serum albumin-containing fraction includes various kinds of contaminants originated from, for instance, biological tissues, cells and blood and therefore, the human serum albumin has been purified by a complicated combination of the foregoing methods. For instance, Japanese Un-Examined Patent Publication No. Hei 5-317079 discloses such a method for preparing human serum albumin comprising the steps of subjecting culture supernatant of human serum albumin-producing recombinant yeast cells to an ultrafiltration treatment, a heat-treatment, a treatment with an acid and an ultrafiltration treatment, in this order and then subjected to a treatment with a cation-exchanger, a hydrophobic chromatography treatment, a treatment with an anion-exchanger and a salting out treatment.
This preparation method is developed to inhibit any coloration of the resulting human serum albumin by heat-treating the supernatant in the presence of a reducing agent. There have also been reported some methods for preparing human serum albumin including the step of a heat-treatment and there have been recognized that a variety of effects can be expected due to the heat-treatment.
For instance, Japanese Examined Patent Publication No. Hei 6-71434 and Japanese Un-Examined Patent Publication No. Hei 8-116985 disclose that a protease is inactivated by heating a human serum albumin-containing culture supernatant prepared according to the gene recombination at a temperature ranging from 50 to 70° C. for 1 to 5 hours in the presence of acetyl tryptophan or an organic carboxylic acid.
Moreover, Japanese Un-Examined Patent Publication No. Hei 7-126182 discloses that microorganisms as impurities are inactivated by heating a recombinant human serum albumin-containing pharmaceutical preparation at a temperature ranging from 50 to 70° C. for not less than 30 minutes.